The invention relates to a current limiter for limiting the short-circuit currents in electric power supply circuits.
In electric power nets, warm (normal temperature) reactors as well as superconductive throttles are used for limiting the current.
A current limiter is activated by providing an ohmic resistance, an inductivity or a suitable combination of the two upon the occurrence of a short circuit. Preferably, the short circuit current is limited by an element that can be triggered in a passive manner, that is, a fault current limiter (FLC).
Superconductors are particularly suitable for that purpose since, during normal operation, they have only a small voltage loss and the transition to normal conductivity results in the establishment of a high electric resistance and causes a high magnetic flux. This can be utilized for a resistive or, respectively, inductive current limiting function (see Prof. Dr. techn. P. Komarek, xe2x80x9cHochstromanwendung der Supraleitungxe2x80x9d, 1995).
A superconductor is not without losses when operated with alternating current. In principle, there are always two loss mechanisms effective, that is, eddy current losses in the matrix and transition magnetic losses (xe2x80x9chysteresis lossesxe2x80x9d) in the hard superconductor itself. With DC operation, the losses in the superconductor are very low.
In the literature, a current limiter is known whose super-conductive impedance is switched by way of rectifiers on the direct current side (Boenig, H. J., and D. A. Paice, 1983, Fault Current Limiter using a Superconducting Coil, IEEE Transactions on Magnetics, Vol. 19, no. 3, p 1051, May). A voltage source is connected at the direct current side in series with the impedance. The voltage source ensures the flowing of a current I0 in the impedance. The value of the current I0 exceeds the amplitude of the alternating current, which flows through the limiter as long as there is no fault but is substantially smaller than a short circuit current. The bridge also permits the load current to flow. In this case, the voltage drop across the limiter equals the voltage drop across the valves in the two branches of the bridge.
In U.S. Pat. No. 3,099,789 or, respectively, FR-No. 1,337,971, an electric circuit is presented which consists of a load, which is connected to an electric energy supply by way of a protective circuit. The protective circuit limits current changes in order to moderate a current change in the load. The protective circuit consists of two conductivities arranged in series and disposed in a parallel circuit with two uncontrolled valves, that is diodes, arranged in series and in opposition to each other. The common points of the diodes and the inductivities are directly connected with one another.
Uncontrolled valves block only currents, which are greater than the response current of the protective device, that is the initial current in the inductivities, and direct the excess currents through the inductivities. The smaller currents, which are also present with alternating currents, are permitted to pass almost uninhibited through the diodes (diode resistance in passing direction). The current flowing through the inductivities cannot follow the net current, which becomes again smaller after the current maximum since it is short-circuited by the diodes and remains essentially constant at the value reached.
The consideration of each following half-wave is divided into currents below the response current and currents above the response current. Smaller currents pass through the conductive diode, whereas the higher currents flow through the inductivities and increase the currents already present in that path. This is repeated with each half-wave for the respective effective part of the circuit. In a short circuit situation, this results in an essential increase of the current flow through the inductivities and in the outer circuits during the time interval which is needed for the shut down of the device (100-150 ms) and particularly during the time interval which is required for securing the selective response of the protective device (1-2 sec).
With the use of active semi-controlled valves such as thyristors in the bridge circuit, the short circuit currents is limited more effectively (Boenig, H. J., and D. A. Paice, 1983, Fault Current Limiter using a Superconducting Coil, IEEE Transactions on Magnetics, Vol. 19, no. 3, p 1051, May; FR-No. 1,337,971). However, in this case, a special apparatus is required which is capable of identifying a short circuit and which changes the control algorithms for the thyristors in the bridge. Consequently, this is not a safely operating current limiter with passive triggering as it is desirable for the present invention. The current limiter is, in this case, not an element that is automatically, passively triggered. The current limiter requires a special control unit for being switched off which substantially reduces the reliability of the current limiter.
During a fault, that is, when the amplitude of the alternating current exceeds the value of I0 of the throttle, the short circuit current is limited by the inductivity of the throttle. The limit for the short circuit current is determined by the value of the maximum energy, which is stored in the throttle during the transient process:
xe2x80x83Wmax=L*Imax2/2
For optimizing the current limiter, the energy (Wmax) stored in the throttle is to be minimized.
It is the object of the present invention to provide a simple current limiter which safely limits the short circuit current in the first 100-150 msec as well as in the following 1 to 2 seconds which is essential for the selectivity of activating the protective device. In short, the response time of the current limiter is to be reduced.
In a current limiter with electric valves for limiting a short circuit in an electric power supply, wherein two throttles are connected in series and in a parallel path with two series connected valves, which are disposed in opposition to each other and the two parallel paths are interconnected by a connection extending between the common potential points of the valves and that of the throttles, the valves are semi-controlled valves, which for the operation of the current limiter are activated by an external control whereby the values are converted from a conductive to a blocking state in which they remain so that the current limiter automatically limits the current upon the occurrence of a fault by which a predetermined current threshold is exceeded.
In a fault situation, that is, when the current exceeds a predetermined value above a design value, the current limiter automatically switches from a conductive to a blocking state without being activated by additional external control signals and it remains in that state.
The two valves may be semi-controlled valves such thyristors.
Preferably, a voltage source is disposed in the connection between the common potential connection of the valves and that of the throttles.
A controllable voltage source is arranged in the respective circuit, which is formed by the inductivity with a valve disposed in a parallel arrangement.
The current limiter is advantageous because, in the event of a short circuit, it is passively triggered limiting the short circuit current during the first 100-150 ms as well as in the following 1-2 sec. In the event of a short circuit, the current is limited by the inductive resistance of the throttles arranged in the alternating circuit. During normal operation, the voltage drop across the current limiter corresponds practically to the voltage drop across the valves.
With the use of semi-controlled valves such as thyristors, the short circuit current is automatically limited without external control signals. Semi-controlled valves change the function of the circuit significantly: After the occurrence of a short circuit, they automatically block within the duration of one or several periods of the alternating current and remain blocked up to the complete shutdown of the faulty circuit. This ensures an effective limitation of the net current by two throttles arranged in series independently of their size and at all times.
Fully controlled valves such as transistors require a special control circuit. They need a separate control circuit, which is involved and also expensive.
During normal operation, a quasi-DC current flows through the current limiting throttles. This results in minimal losses in these elements.
The current limiter according to the invention has the following advantages:
Small size;
Small power net and internal resistance and small short circuit currents;
The arrangement and operating means need to be dimensioned for only small short circuit currents;
Substantial increase of the reliability of the current limiter;
Increased life of the plants and operating means;
Low effects at the fault location;
Increased effectiveness in limiting short circuit currents;
Low losses during normal operation.
Below, the invention will be described in greater detail on the basis of the accompanying drawings.